There are basically three types of cable faults, which should be distinguished from one another. A first type is a ground fault in which one or more conductors of the cable are connected with ground potential through a faulty conductive connection. A second type is a short circuit in which two or more conductors of the cable are connected with one another in a high-resistance or low-resistance manner. A third type is an interruption or break in which one or more conductors within the cable are interrupted. There are also fault modes that involve combinations of these fault types.
Locating of such faults can involve pre-locating and post-locating, or gross locating and exact locating, which should also be distinguished from one another. Pre-locating involves determining a more or less exact distance of the fault location from a cable beginning and, if applicable, an identification of the faulty cable. Post-locating involves a point-exact localization of the fault location.
Depending on the type of the cable fault, various different methods are known for the pre-locating and post-locating. An essential underlying principle of such methods is the use of reflection measurements in which an electrical pulse is fed into the cable or conductor line, propagates along the line and is then reflected at the fault location. The reflected pulse is received back at the cable end at which the pulse was fed into the line. Thereby the distance from the cable end at which the pulse was fed into the line to the fault location can be calculated based on the time duration between the infeeding of the pulse and the receiving of the reflected pulse, and therewith its transit time. Moreover, with higher energy pulses, an electrical arc-over or a sparking arc can be produced at the fault location, and this leads to an acoustic discharge noise, which can be detected or recorded by an acoustic sensor on the ground surface above the buried cable. By evaluating the varying intensity of the detected acoustic signal at different locations on the ground surface, e.g. as the acoustic sensor is moved along the ground surface, thereby the fault location can be localized. Furthermore, it is also possible to evaluate, at a measuring location, the running time or transit time of the acoustic signal and especially the time duration between the detection of the electromagnetic field produced by the electrical pulse and the detection of the acoustic signal triggered by this pulse.
In that regard it has been shown, however, that the actually received acoustic signal (hereinafter called the “received signal”) not only contains the acoustic signal produced at the fault location of the cable, but rather also can contain considerable acoustic interference noises caused by e.g. environmental influences (wind, rain) or the street traffic or persons and the like. These interference noises interfere with identifying, or make it significantly more difficult to identify, the acoustic signal produced at the fault location of the cable, and thereby can impair the accuracy and efficiency of the fault locating.